In the course of the fabrication of integrated circuits (chips) the wafers must, after certain operations, inter alia after etching, be rinsed with various chemicals and with highly pure water. The reclaim, produced in large amounts, is of a quality, owing to the content of organic and inorganic substances, which makes any direct recycling into the ultra-pure water production circuit impossible. On the other hand, recycling of the reclaim is considerably impeded by the organic contaminants arising from semiconductor fabrication, since it is necessary to achieve TOC values (Total Organic Carbon content) of less than 5 ppb in the ultra-pure water circuit, and these organic contaminants can, as a rule, be removed only inadequately by means of conventional processes such as reverse osmosis, mixed-bed filters, devolatilization, UV irradiation and ultrafiltration.
Typical semiconductor reclaims contain inorganic and organic substances such as, for example, fluoride, chloride, nitrate, sulphate, phosphate and ammonium ions, hydrogen peroxide, isopropanol, acetone, N-methylpyrrolidone, tetramethylammonium hydroxide, methanol, ethanol, butanol, acetic acid, dimethyl sulphoxide, propylene glycol methyl ether acetate and the like. The main constituents usually comprise isopropanol, acetone, N-methylpyrrolidone, hydrofluoric acid, hydrochloric acid, sulphuric acid, phosphoric acid, hydrogen peroxide, ammonia, ammonium fluoride, tetramethylammonium hydroxide and the like. The reclaims may typically have an electrical conductivity of from about 10 to 2,000 .mu.S/cm and a TOC value of from about 0.1 to 20 ppm. The pH is generally between about 2 and 9, usually below 7.
On the grounds of cost and environmental protection and on the grounds of water scarcity it is desirable for these reclaims, whose overall contaminant content as a rule is distinctly below that of ordinary tap water, to be treated to the extent of them being usable once more in semiconductor fabrication.
The treatment of the reclaims at present makes use of ion exchange processes, reverse osmosis processes, adsorption on activated carbon, biological processes and ultrafiltration. For example, free mineral acids and organic acids are removed routinely via weakly or strongly basic anion-exchange resins. Furthermore, reverse osmosis installations which, in some cases, are already present in the make-up water treatment systems of the ultra-pure water installations are as a rule quite able, together with the existing ion-exchange stages, to remove both inorganic and organic acids, bases and salts to an adequate degree. Hydrogen peroxide and hydrophilic organic compounds such as isopropanol, acetone, N-methylpyrrolidone, methanol, ethanol, butanol, dimethyl sulphoxide and the like, on the other hand, are usually removed only in amounts of about 50-70%, even in reverse osmosis stages, falling far short of the demanded TOC values of less than 5 ppb. In principle, it is possible for the concentrations of these compounds to be further reduced by additional treatment with activated carbon, but even in this manner it is often impossible or at least extremely difficult to achieve satisfactory separation.
The known processes therefore have the drawback that a number of purification methods have to be combined and that nevertheless, particularly in the case of comparatively high concentrations of organic contaminants, satisfactory results often cannot be achieved, and/or biofouling may adversely affect the stages downstream of a biological stage or an activated-carbon filter.
On the other hand, U.S. Pat. No. 4,040,990, JP-A-62/197308, U.S. Pat. No. 4,839,331, EP-A-0604110 and EP-A-0623557, for example, disclose carbonaceous adsorbents which are obtained by pyrolysis of synthetic polymers and, compared with activated carbon, have a more hydrophobic surface and are therefore able to adsorb hydrophobic organic compounds, in particular hydrocarbons and halogenated hydrocarbons, more effectively than activated carbon. Such products are commercially available, e.g., under the brand name Ambersorb (Rohm and Haas, Philadelphia, USA).
EP-A-0285321 further proposed the use of pyrolysates of cross-linked polymers as adsorbents to remove bacterial endotoxins (lipopolysaccharides), which may be present as pyrogens in tap water or in purified water, e.g. as a result of storage. Suitable cross-linked polymers are, for example, styrene-divinylbenzene copolymers which, if required, may have been sulphonated or chloromethylated and then aminated to give an ion-exchange resin. As EP-A-285321 further discloses, this method can also be used in water treatment processes in which highly pure water is produced and stored, by means of the stored water, prior to a subsequent filtration step, being brought into contact with the pyrolysate. In this case, however, the starting material for producing ultra-pure water is tap water, and the adsorption step serves only for the removal of any pyrogens present from the stored highly pure water.